In her final message before the Commons showdown began, the PM admitted she must attempt to renegotiate the Withdrawal Agreement, and appealed for MPs to vote for an amendment that would give her a "mandate" to demand the Irish border backstop is replaced.
The amendment calls for the backstop to be replaced with unspecified "alternative arrangements" to avoid the reintroduction of border checks in Ireland, and said Parliament would support Mrs May's Brexit deal if this change were made.
One Tuesday night, in a series of votes on parliamentary amendments, they will set out what they want her to do next. The backstop would keep the U.K.in a customs union with the EU in order to remove the need for checks along the border between the U.K.'s Northern Ireland and EU member Ireland after Britain leaves the bloc.
British MPs seeking to pressure Prime Minister Theresa May this week to renegotiate her Brexit deal should heed European Union warnings that it will not succeed, Ireland's foreign minister warned on Sunday.
"There will be no more negotiations on the Withdrawal Agreement", said Weyand, a German senior civil servant at the European Commission, reiterating the EU stance.
Meanwhile, Labour has yet to reaffirm their support for any amendments but if anything they will be supporting the Cooper Amendment (which could be a close call as well).
But Labour announced it would throw the party's support behind an opposition amendment to push Brexit day back from March 29 to the end of this year and put Parliament in the driving seat on the way forward.
"The Brady amendment will still fail, but by a much smaller margin than the original bill, which will give her something to go back to Europe with", said RBC's Cole.
At least now we know where most MPs stand on this issue, but it doesn't really change anything: it's not as though they've suggested how they hope to avoid leaving without a deal.
MPs then backed an amendment from Tory Sir Graham Brady, supported by Mrs May, calling for legally-binding changes to the deal.More news: Measles outbreak prompts state of emergency declaration by Washington governor
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Parliament's response to May's call to "send an emphatic message" to the European Union is likely to leave the bloc even more confused about British aims. Parliament has expressed a desire to stop no-deal - but I am concerned that without legislation in place its probability has increased.
British stocks jumped Tuesday and the pound was firmer ahead of the votes.
Labour will not formally say if it will order its MPs to back the plan until House of Commons Speaker John Bercow announces which amendments have been chosen for consideration on Tuesday morning, but three people familiar with the leadership's thinking said it was minded to support it.
May's approach drew praise from Brexit-backing lawmakers but prompted scorn from their pro-EU colleagues.
If Theresa May backs the Steve Baker/Nicky Morgan Brexit plan C she probably wins the Brady amendment and defeats Cooper/Boles - but doing so would be based on what she believes to be a lie, since she is (probably rightly) convinced there is zero chance of European Union 27 leaders agreeing on the Morgan/Baker Brexit plan.
The backstop is a type of insurance policy aimed at preventing a hard border between the Republic of Ireland and the British province of Northern Ireland if no other solutions can be agreed.
Reacting to the result, Jeremy Corbyn chose to focus on the success of another no-deal amendment, in the name of the Conservative backbencher Dame Caroline Spelman, which was carried by 318 votes to 310.
The government hopes to bring the deal back for a new vote in Parliament in February, with enough changes to reverse its thumping defeat on January 15, when lawmakers rejected it by 432 votes to 202.
Ireland has said it doesn't want any changes to the backstop.
Overnight, a report by major think tank the Centre for European Reform's (CER) revelaed that while Brexit has still not happened yet, it has already caused the economy to be 2.3% smaller than if Brits had voted to remain in the EU.